This invention relates to a method and apparatus for longitudinally seam-welding steel pipes manufactured by a U-ing/O-ing/expanding process which is abbreviated as an UOE process. This seam is generally produced by the use of a seam welder by welding the inner surface of the longitudinal groove defined by the welded edges of a steel pipe and then the outer surface of the longitudinal groove.
According to the prior art submerged arc type steel pipe seam-welding apparatus shown in FIG. 1, a boom or horizontal support arm 12 is carried by a traction carriage 13 and guide carriage 14. A welding head 11 is fixed to the front end of the arm 12. The welding head 11 is positioned over a portion of the longitudinal groove 15 of a steel pipe 16 which is to be welded. A flux is applied to the welding spot. An electrode wire 17 is conducted by feed rollers 18 through the welding head 11 to the welding spot. A large alternating current is supplied from a terminal of a power source 19 to the electrode wire 17 through an electrical insulative covered conductor 20 (hereinafter referred to as "cable 20") and welding head 11 to produce arcs between the leading end of the electrode wire 17 and the welded spot of the steel pipe 16. Where, under this condition, the traction carriage 13 is hauled, a longitudinal seam 21 is formed on the inner surface of the groove 15. The steel pipe 16 is connected to the other terminal of the power source 19 through a line 22. After an inner seam 21 is formed on the inner surface of the seam 15 of the steel pipe 16, an outer seam is produced on the outer surface thereof by a process similar to the above-mentioned process, thereby completing the seam-welding process.
The prior art submerged arc welding method utilizing an alternating electric current has a drawback that the properties of the steel pipe are much deteriorated due to the fact that a large heat input arises when the longitudinal seam is formed on a steel pipe.
In order to overcome this disadvantage, there has been proposed a method and apparatus for longitudinally seam-welding steel pipes wherein a large direct electric current with a low voltage flows through a cable to produce an arc with a moderate amount of heat input as shown in U.S. Pat. No. 4,071,732.
In the case, however, where a seam is produced particularly on the inner surface of the longitudinal groove of a steel pipe by such an direct current seam-welding method, a large current running through the cable 20 gives rise to the creation of a strong circular magnetic field around the cable 20.
Thus, a magnetic circuit is generated along the periphery of the steel pipe 16 having a high magnetic permeability and the surfaces of the groove 15 are magnetically saturated. As a result, leakage flux is produced in the groove 15. During the direct current seam welding process, however, an arc which is a flow of electrified particles lying within the range of the leakage magnetic flux is inevitably deflected in a direction along the groove 15 in accordance with Fleming's left-hand law, leading to the so-called magnetic blow. The plasma jet stream produced at the forward end of the electrode wire acts as a dynamic pressure on the molten metal and irregularly vibrates the molten pool. This results in the ocurrence of welding defects such as undercut of bead, humping bead and lack of fusion of base metal. In addition, the arc is produced at one lateral side of the electrode wire, leading to the melting of said lateral side. Accordingly, molten droplets fail to smoothly transfer from the electrode wire to the base metal, thereby creating coarse spattering. Further, the above-mentioned drawbacks are also encountered in the production of an outer seam on the outer surface of the groove of a steel pipe.
It is accordingly the object of this invention to provide a method and apparatus for seam-welding a steel pipe, wherein electric current is made to run through a conductive enclosure tube surrounding the electrical insulative covered conductor (i.e., cable) in a direction opposite to that in which an electric current is conducted through the cable to prevent the steel pipe from being magnetized.